1. Field of the Invention
The present invention relates to an image input apparatus which includes an image reading circuit positioned below a placement surface on which an object to be examined is placed, and inputs an image of the object by reading the object by an image reading device.
2. Description of the Related Art
A fingerprint has a pattern unique to an individual and hence is a very useful means for authenticating a person. Recently, a fingerprint authentication apparatus which applies a fingerprint to personal authentication is developed. More specifically, this fingerprint authentication apparatus performs personal authentication by comparing a fingerprint image read by an image reader with fingerprint image data of a preregistered person. It is being attempted to mount this fingerprint authentication apparatus in information apparatuses such as a personal computer, PDA (Personal Digital Assistance), and cell phone.
Jpn. Pat. Appln. KOKAI Publication No. 2002-94040 describes a two-dimensional image reader to be used in the fingerprint authentication apparatus. This two-dimensional image reader includes a photosensor array in which a plurality of photosensors are arranged on a transparent substrate, a backlight positioned at the back of the photosensor array so as to oppose its rear surface, a transparent electrode layer covering the surface of the photosensor array, and a sensor for sensing a voltage change in the electrode layer. The operation of the conventional two-dimensional image reader and a method of using it will be explained below. When a person to be examined places his or her finger on the electrode layer, the sensor senses a unique voltage change caused by the contact of the finger with the electrode layer, since the person himself or herself has a unique resistance and capacitance. In accordance with this finger sensing by the sensor, the backlight illuminates the finger with light, and the photosensor array reads an image of the finger by an image reading operation. This finger image read by the photosensor array is represented by the intensity distribution of reflected light from projections and recesses of the finger on the contact surface.
A finger perspiration state differs from one person to another, and a pressure with which a finger placed on the electrode layer presses the electrode layer also differs from one person to another. Different finger perspiration states cause different unique voltage changes when these fingers come in contact with the electrode layer. Similarly, different electrode layer pressing forces of fingers cause different electrode layer voltage changes because the contact areas between these fingers and the electrode layer are also different. In the conventional two-dimensional image reader, therefore, if the tolerance of variations in perspiration state between individuals and the tolerance of variations in finger pressure are too small, a finger of a certain person placed on the electrode layer may not be sensed by the sensor and so a fingerprint image of this person cannot be read, while a finger of another person placed on the electrode layer can be sensed by the sensor and so a fingerprint image of this person can be read. The perspiration state or pressing force of a finger of even the same person may change in some cases, so a fingerprint image of the finger cannot be read in this case. Also, if the tolerance is too large, an operation error may be caused by an object, other than a finger, which comes in contact with the electrode layer. Likewise, if the tolerance is extended to cover even weak electrode layer pressing forces, projections of a finger do not come in close contact with the electrode layer. Since the pattern of the fingerprint cannot be clearly read in this case, accurate authentication may not be performed. Furthermore, such very small voltage changes cannot be accurately sensed owing to noise such as ambient electromagnetic waves.